It has been extensively used a lithium secondary cell which uses an organic solvent, absorbs and releases reversibly lithium ions on positive and negative electrodes and allows repetitive charge/discharge for applications such as potable electronic devices or personal computers, and even in a battery for driving a motor in hybrid electric vehicles. There exists a need for advanced miniaturization and weight lightening in such a lithium secondary cell, as well as there exist major challenges such as increased amounts of lithium ions absorbed and released in positive and negative electrodes, improved energy density and reduced cycle deterioration in charge/discharge.
This type of lithium secondary cell has a structure configured by opposite-disposing a negative electrode active material layer having a negative electrode active material formed on a current collector and a positive electrode active material layer having a positive electrode active material formed on a current collector with a separator intervened therebetween within an outer body wherein the negative electrode active material and the positive electrode active material are capable of absorbing and releasing reversibly lithium ions in charge/discharge, filling an electrolytic solution to immerse them, and sealing the outer body. The active material of the negative electrode active material layer or the positive electrode active material layer is bound intimately by a binder, and these layers are formed adhered to the respective current collector.
As such a lithium secondary cell, it has been reported one having a negative electrode formed by intimately binding a carbon powder negative electrode active material with a polyimide binder (Patent Documents 1 and 2), one having a negative electrode formed by intimately binding a silicon or silicon alloy negative electrode active material with a polyimide binder (Patent Documents 3 and 4) as well as one having a negative electrode formed by intimately binding a silicon oxide negative electrode active material with polyimide (Patent Document 5).
However, as a binder for active materials, polyimide having the following structural formula is used.

Such polyimide undergoes the ring-opening of imide rings during repetitive charge/discharge and hence exhibits reduced chemical resistance, resulting in shortened cell life. Accordingly, there exists a need for a binder of which the deterioration of chemical resistance involved in charge/discharge may be suppressed.